Interface NodeListener

Listener to special changes in Nodes. Is a property
change listener so that all changes in properties in the node can be fired
in the usual way.

Methods childrenAdded, childrenRemoved and childrenReordered are called
with Children.MUTEX.writeAccess which guarantees that no other thread
can change the hierarchy during that time, but also requires proper
implementation of all NodeListeners which should avoid calls
to other threads which might require access
to Children.MUTEX due to changes nodes hierarchy or do any other kind of
starvation.